Access Type

Open Access Dissertation

Date of Award

January 2017

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Guru P. Dinda

Abstract

Additive manufacturing (AM) of metals is finding numerous applications in automotive and aerospace industry. In 21st century, aluminum is second to steel in automotive sector, because of its high strength to weight ratio. In current era, casted components of Al-Si alloy and high strength Al 7xxx series alloys are being used from low load to high load components. The primary study of this research was to achieve the defect free deposition of Al-Si and Al 7050 alloy. Al-Si alloys samples have been manufactured at lab scale using various additive manufacturing processes, but so far there is no literature available to investigate the feasibility of fabricating Al-Si alloy component for automotive component applications using Direct Laser Metal Deposition (LMD) technique. This research deals with the practical challenges of building single wall and block deposition (cuboid shapes) of eutectic Al-Si alloy using direct laser metal deposition process for automotive applications. Microstructural investigation using optical and scanning electron microscopy confirms greater than 99% component density of as-deposited Al-11.2Si and Al 7050 sample. Tensile test sample extracted from Al-11.2Si deposit showed an impressive elongation of 9% with an ultimate tensile strength of 225 MPa. This investigation revealed that direct laser metal deposition could successfully print the eutectic Al-Si alloy bracket on shock tower hood without any distortion or bending. For Al 7050, parameter optimization (laser power, powder flow rate, and scanning speed) gets difficult with the presence of various low melting and boiling point alloying elements such as Zn, Mg etc. Microstructural characterization and microhardness test were performed and results showed microhardness of 100 HV of as-deposited sample and 128 HV after heat treatment. To cope with the issue of vaporization of second phase elements Mg and Zn, laser metal deposition of Al 7050 alloy powder coated with nickel was conducted using optimized parameters. Microstructural investigation using optical and electron microscopy revealed that the deposits are free from relevant defects such as porosity or lack of fusion. However, the added nickel was partially segregated in the inter-dendritic boundaries and formed brittle Al3Ni intermetallics. As a result, as-deposited Ni coated Al 7050 alloy showed almost no tensile ductility. Laser deposited samples were friction stir processed to refine and uniformly distribute Al3Ni particles in the α-Al matrix. Tensile test results revealed a good combination of yield strength (178 MPa), UTS (302 MPa), and 6% elongation of friction stir processed (FSP) samples. Post FSP heat treatment additionally improved both strength and elongation about 10%. Microstructural investigation revealed a systematic change of columnar to equiaxed dendrites from bottom to top of each deposited layer. Laser metal deposited Al 7050 alloy exhibits different composition with reduced Mg and Zn quantity than it was present in Al 7050 powder due to evaporation of Mg and Zn. This research also presents the technique to study ten different Al 7050 compositions in one coupon of 20 mm ×20 mm. Final compositions were measured using EDS to establish correlation between Mg content in feed powder and in laser deposited material. Microhardness showed linear increment trend with linear addition of Mg by weight percent. Formation of strengthening precipitates Al3Mg2 and MgZn2 were escalated with the linear addition of Mg from bottom to the top of the Al 7050 gradient sample. Vaporization curve showed that with the addition of more Mg, respective loss also increases as compared to loss of Mg happened in previous layer. Microhardness test revealed that with addition of Mg, microhardness increased from 94 HV to 154 of as-deposited and 110 HV to 190 of heat-treated Al 7050 gradient sample. This technique is very useful to develop new alloys by studying various compositions in one sample saving time and effort.

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